[ANCM In-proc] Improve handling of malformed warmup requests

Epic https://github.com/aspnet/AspNetCore/issues/8833

In some versions of Windows (RS1 or below), we get a malformed warm-up request that has missing data. We throw NullReferenceExceptions in that case, which is not ideal. We should investigate this further and try to make our logic more resiliant to these nulls, but only during these malformed warmup requests.

[analogrelay]

When this is done, consider backporting.

[jkotalik]

Acceptance checklist (check one item)

• We decided not to take this fix.
• The fix is tests-only.
• The fix contains product changes (check all items below).
  • Relevant XML documentation comments for new public APIs are present.
  • Narrative docs (docs.microsoft.com) are updated. (check one item below)
    • The change requires a new article. An issue with an outline has been filed here: [ISSUE LINK]
    • The change requires a change to an existing article. A closed docs PR with these changes is linked here: [PR LINK]
    • The change requires no docs changes.
  • Verification has been completed. (check one item below)
    • The change is in the shared framework and was verified against the following versions
      • SDK installer: 3.0.100-preview6-012113
      • ASP.NET Core Runtime: 3.0.0-preview6-19279-01
      • .NET Core Runtime: 3.0.0-preview6-27728-04
    • The change is in an OOB NuGet/NPM/JAR package and was verified against the following version of that package: [PACKAGE ID] [VERSION]